This application claims the priority of Korean Patent Application No. 2002-46564 filed on Aug. 7, 2002 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly, to a wavelength tunable vertical cavity surface emitting laser diode (VCSEL).
2. Description of the Related Art
A VCSEL has a structure in which distributed Bragg reflectors (DBRs), each having a refractive index of 99%, are formed over and under an active layer to resonate light in a direction to perpendicular to stacked layers. The DBR is formed by stacking materials having a great difference in a refractive index and similar lattice constants so as to be epitaxially grown, e.g., by alternatively stacking GaAs and AlAs or by alternatively stacking a pair of dielectric materials, having a great difference in a refractive index, selected from dielectric materials, such as a silicon oxide layer SiO2, an aluminum oxide layer A12O3, and a titanium oxide layer TiO2. In such a DBR, it is preferable that an energy bandgap is greater than an oscillation wavelength to prevent light from being absorbed, and the greater a difference between refractive indexes of two materials constituting the DBR, the better.
FIG. 1 is a cross-sectional view of a conventional VCSEL having a DBR. Referring to FIG. 1, a lower DBR 21 is formed on a substrate (not shown). The lower DBR 21 is formed by alternatively stacking two materials 21a and 21b having relatively different refractive indexes. An n-contact layer 22 is formed on the lower DBR 21. A cavity resonator layer C, which is a stack of an n-lower carrier limiting layer 23, an active layer 24, and a p-upper carrier limiting layer 25, is formed on the n-contact layer 22. A current constricting layer 26, which limits the inflow of a current for laser oscillation, is formed on a predetermined region of the cavity resonator layer C, and a dielectric layer 26a is formed on a remaining region of the cavity resonator layer C. The current for laser oscillation flows into the active layer 24 only via the current constricting layer 26. A p-contact layer 27 is formed on the current constricting layer 26 and the dielectric layer 26a. An upper DBR 28 is formed on a predetermined region of the p-contact layer 27. Like the lower DBR 21, the upper DBR 28 is formed by alternatively stacking two materials 28a and 28b having different refractive indexes. Lower and upper electrodes 30 and 29 are formed on edges of the n-contact layer 22 and the p-contact layer 27, respectively.
As such a VCSEL has been used in optical communications or optical recording media employing a next generation wavelength division multiplexing method, the need for a wavelength tunable VCSEL has been increased. The simplest method of realizing the wavelength tunable VCSEL is to vary a resonance width of the wavelength tunable VCSEL so as to vary a wavelength of laser light emitted from the wavelength tunable VCSEL by heating or cooling the wavelength tunable VCSEL.
However, if a wavelength of emitted light varies by varying an operation temperature, since it is difficult to rapidly vary the operation temperature, it is also difficult to rapidly vary the wavelength of the emitted light. In addition, if the operation temperature increases, laser oscillation characteristics may be sharply reduced due to excessive gain loss, thereby limiting a variable range of the operation temperature. Also, the VCSEL may stop operating due to sudden discharge during the variation of the operation temperature.